1. Field of the Invention
The present invention belongs to the technical field of methods of heat treating (annealing) a crystal of a fluoride such as calcium fluoride, barium fluoride, strontium fluoride, or the like.
2. Related Background Art
The photolithography is preferably used for production of semiconductor devices such as microprocessors, memories, system LSIs, image sensors, light emitting elements, display elements, and the like. In the field of optical processing such as the photolithography, there has been used an ultraviolet light more frequently. This increases a need for glass materials other than quartz glass, for use in optical parts such as lenses, prisms, halfmirrors, window members, or the like. The fluoride crystal such as of calcium fluoride, barium fluoride, strontium fluoride, lithium fluoride, or the like is preferable as a glass material for optical parts which are required to have a high transmittance.
In the production steps of a fluoride crystal, there is carried out as desired an annealing processing in which the fluoride crystal is heated from room temperature up to a desired temperature and then allowed to be cooled to room temperature, thus relaxing a stress generated inside the crystal.
When the birefringence (double refraction) of a fluoride crystal is measured before and after carrying such an annealing processing, the fluoride crystal will be reduced in birefringence after the annealing.
However, in order to fully exhibit the effect of annealing, annealing processing for a very long period of time is necessary, which has been responsible for increase of the production cost of a fluoride crystal.
An object of the present invention is to provide a method of heat treating a fluoride crystal that can be carried out in a short period of time and can fully suppress the birefringence.
According to the present invention, there is provided a heat treating method of heat treating a fluoride crystal comprising the steps of: raising the temperature of a fluoride crystal; reducing the thus raised temperature of the fluoride crystal at a first temperature reducing rate; and then reducing the temperature of the fluoride crystal at a second temperature reducing rate which is larger than the first temperature reducing rate.